Derivative Activities - Effects of Non-Hedge Derivatives on Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative Fair Value Income (Loss)	$ 126,312	$ 202,004	$ (34,791)	$ 122,839	$ 412,422	$ 142,057	$ (24,109)	$ (146,850)	$ 416,364	$ 383,520	$ (61,825)
Swaps
Derivative [Line Items]
Derivative Fair Value Income (Loss)									398,020	367,484	(50,526)
Re-purchased Swaps
Derivative [Line Items]
Derivative Fair Value Income (Loss)									851	0	1,323
Collars
Derivative [Line Items]
Derivative Fair Value Income (Loss)									16,539	42,836	(16,062)
Basis Swaps
Derivative [Line Items]
Derivative Fair Value Income (Loss)									$ 954	$ (26,800)	$ 3,440